The invention relates to a reactor for chemical reactions, especially to a polymerization reactor and in particular to a reactor for emulsion, suspension, solution and/or bulk polymerization.
The preparation of polymers by liquid phase polymerization is customarily subdivided into bulk, solution, precipitation, suspension and emulsion polymerization, the latter two techniques differing from the others in starting from an already two-phase initial mixture. The polymerization reaction is conducted either in a batch reactor, in a continuous flow pipe, in a cascade of stirred vessels or in a continuous stirred-vessel reactor. Among these, vessel reactors have acquired the greatest importance in the chemical industry since they possess great flexibility in terms of operating conditions and mode of operation and can be adapted to virtually all process requirements. Stirred-vessel reactors are suitable for discontinuous and continuous operation and possess a broad scope of application ranging from laboratory vessel to large-scale reactor. Stirred-vessel reactors are available in standardized constructions in a very wide variety of materials and combinations of materials for numerous applications.
In addition to the customary cooling and heating devices and incoming and outgoing lines for starting materials and products, the reactor vessels have stirring devices, usually consisting of a stirrer driven by way of a stirrer shaft, and for certain applications also have stators which act as flow disruptors for better mixing. The stirrers themselves are fastened to usually vertical stirrer shafts which project either from above or from below into the generally cylindrical reactor vessel. Central installation from above into the reactor vessel is generally preferred in accordance with the prior art, since it is then relatively easy to seal off the stirrer shaft. A disadvantage is that, because of the bending moments which occur, the stirrer shaft must have a relatively large diameter. Introducing the stirrer from below into the reactor, on the other hand, reduces the stirrer mass but requires more complex sealing of the stirrer shaft at its point of penetration through the reactor floor.
It is known to achieve many different tasks in process engineering with the aid of stirring devices. Examples of such tasks include dispersing, homogenizing, mixing, gassing, heat transfer, suspending and dissolving. Depending on parameters specific to the system, such as viscosity, number of phases, density and differences in density, particle sizes, shearing, heat production and reaction temperature, there are complex theoretical relationships of heat transfer and fluid dynamics, and a large number of specially constructed stirrer forms and reactor forms which have already been proposed is intended to address these relationships. To enable the processes to be conducted more safely, specific techniques have also been employed, such as semicontinuous supply of the substance streams, whose intention is to provide better control of the liberation of heat. The literature includes a large number of recommendations which provide for each problem that occurs an appropriate solution in the form of particular designs of apparatus (for example, stirrer type), specification of parameters (for example, rotary speed or dimensioning) or formulations and particular measures (for example, use of solvents for controlling viscosity and as heat transfer medium, control of polymerization temperature and/or of the addition of the substance streams). Regarding further studies in the technical aspect of stirring in reactors, the accompanying phenomena of heat transfer and the knowledge of flow processes, reference is made to the literature cited at the end of the description.
In [11] (see the list at the end of the description) the introduction (pp.282-285) gives some examples of industrial reactors and the problems which occur in the course of their operation.
[17] investigates the effect of some technological parameters on the course of the emulsion polymerization of the styrene-butyl acrylate-acrylic acid system. It was found that the addition of the monomer emulsion by way of a pipe through the lid of the reactor into the space between the bottom blade of the stirrer and the floor of the reactor prevents the formation of coagulum. On the other hand, however, an increase in the amount of residual monomer, and a greater variation in particle size, implying poorer reproducibility, were observed.
Once a reactor has been installed it is frequently used to prepare a large number of different products. This means that the system is generally optimum only for the design case and must be modified if a different product is to be prepared. Among the known polymerization reactors and stirring devices it is true that there are already embodiments which permit conversion to a different polymerization process, for example by replacing the stirring blades, with relatively little difficulty. However, there continues to be a lack of constructions which enable the conversion and cleaning of the polymerization reactor to be carried out much more simply, more rapidly and thus more cost-effectively when changing over to a different production process.
It is also already known to instal stirrers from below into the reactor (see DE 44 21 949, column 3, lines 7-18 [13]). FIG. 1 of DE 44 21 949 also reveals a discharge port on the floor of the reaction vessel. The feed flows are introduced into the reactor from above. A reactor of this kind is relatively difficult to clean.
It is an object of the present invention, therefore, to provide a reactor for chemical reactions which permits approximation to the process optimum for the particular process, e.g. polymerization process, being conducted without great expenditure in terms of time and money. Such a reactor should, moreover, feature high flexibility in respect of switching the plant over to other processes.
We have found that this object is achieved, surprisingly, by a reactor where the stirring mechanism and the incoming and outgoing lines are installed on the reactor floor.
The term stirring mechanism refers here to a stirring device comprising one or more than one stirrer.
The invention accordingly relates to a reactor for chemical reactions, in particular to a polymerization reactor, having a stirring mechanism, incoming and outgoing lines and removable lid, wherein both the stirring mechanism and incoming and outgoing lines are installed on the reactor floor.
The terms incoming and outgoing lines refer both to lines for streams of substances such as starting materials or products and to electrical and other lines for, say, temperature sensors, pH measuring equipment, pressure measuring equipment, concentration determining probes, instruments for optical analysis, or other on-line analytical instruments.
The advantages achieved with the novel reactor are many and various:
After removal of the lid, the interior of the reactor is readily accessible. This facilitates a cleaning operation and enables stirring blade changeover to be carried out easily and quickly. The cleaning of stirring blade, stirrer shaft, internals and especially the inner wall, floor and lid of the reactor to remove wall deposits and baked crusts, which depending on the composition of the polymer can be very hard or very soft or visco elastic, and can be several centimeters thick, is very easy if the lid can be taken off and removed to the side. This is particularly important in the case of the emulsion polymerization of shear-sensitive products, since in this case there are particularly thick deposits in high-shear regions, for example on the stirrer or internals, which greatly reduce the passage of heat and which impair the function of the sensors (for example, temperature control).
The possibility afforded by the invention for rapid and simple changeover of the stirring mechanism is a significant advantage, since the different products, formulations or mixtures prepared and processed in a polymerization reactor differ in viscosity, sensitivity to shearing, soiling tendency and color. Another example is the preparation of polymer dispersions which, for example, have particular adhesive groups whose purpose when applied, for example, to metal is to produce effective adhesion to substrates. Products of this kind naturally adhere very well to the reactor wall too, leading to an increased need for cleaning.
Customary stirrers are single-stage or multistage embodiments, especially those which produce not only a tangential flow component but also an axial flow field. Preferred stirrers are those having 1 to 7 stirring blade stages attached, preferably equidistantly, on the axial stirrer shaft, examples being blade, anchor, impeller, Pfaudler, disk, helical, bar, finger, propeller, sigma, paddle, pitched-blade and coaxial stirrers and also corresponding multistage stirrers, such as cross-arm, multiflow, multipulse countercurrent (Mehrimpulsgegenstrom, MIC), INTERMIG and INTERPROP stirrers (cf. [4]). With the multistage stirrers it is possible, by altering the distance between the stirrer stages and/or their relative angles, to exert appropriate influence on the requirements of fluid dynamics and on the problems of heat dissipation of the particular reaction. The literature recommends various stirrers for different viscosities, flow conditions and heat release. An overview of stirrer types can be found, for example, in [4] on page RS2,3 and [7] on page 92 ff. Double blade stirrers with various constructional examples are described in [14] and [15].